DE3816188C2 - - Google Patents

Abstract

To counteract in particular the occurrence of acetaldehyde on a hot runner (20) for the manufacture of preforms from a PET material, the material advanced in channels (26) is subjected to an additional vortexing. For this purpose, the channels (26) have inserted in them a component (32), preferably of metal, which is provided with radial sets of spokes (14a), which are offset in each case azimuthally with respect to one another in the axial direction. Along with the vortexing mentioned, these sets of spokes also act as homogenising elements for the temperature distribution in the flow cross-section of the material advanced in the channels. <IMAGE>

Description

The invention relates to a static mixer for thorough mixing a flowing liquid with a tubular Pipe through which the liquid flows and which preferably has a circular inner cross section and with at least one substantially radially oriented pin-like flow divider in the line.

Static mixers are known. With them becomes a fluid or viscous good in a line section for that Well by a multitude on one axial section of the line, e.g. B. mixed helically arranged slats. As a result, the flow through the line section is experienced A good swirl, but with the mixer fins on the one hand a significant increase in flow resistance cause, on the other hand, such mixers in construction and are complicated and expensive to manufacture.

The above-mentioned version of a static mixer (US-PS 40 49 241) points radially into the flow divider  Line protruding pins with one end are attached to the wall and close to the opposite Wall sufficient. The pins are in the direction of flow employed and otherwise on a helix arranged. The lead and the pins exist primarily made of plastic.

In many applications, it's not just about the bare Mixing or maintaining a homogeneous state of the liquid, but also about getting an even Generate or maintain temperature distribution in the mix, being between the line wall and the Liquid usually has a temperature gradient.

The invention has for its object a static To design mixers of the structure mentioned at the beginning that with low flow resistance not only within a homogeneous temperature distribution, but heat transport between the pipe and the mix is also achieved becomes.

This object is achieved in that the flow divider is made of a highly thermally conductive material and thermally connected to the wall of the pipe is to measure temperature differences between the wall and the Break down liquid.

The invention is particularly suitable for viscous liquids mix with low flow resistance or in keep homogenized state. At the same time takes place over the Flow dividers due to their thermal conductivity and their thermally conductive connection with the line a constant Heat transfer from the line into the liquid instead.

The pin-like flow dividers are preferably spoke-like arranged, for example three spokes  can have and advantageously made of metal, in particular are made of steel.

The flow divider can be attached to the wall of the pipe or be inserted into the line in a press fit.

According to a further preferred embodiment, that two azimuthally offset spoke sets are provided, which are centrally connected to each other at a distance.

This design results in particularly good mixing with effective heat transfer at the same time, but without - especially with viscous liquids - the flow resistance gets too high. Such a one-piece can also be Flow divider with two spaced sets of spokes easy, especially afterwards in a line deploy.

The invention further relates to a hot runner block for Tempering and distributing a plastic melt higher Temperature on a variety of forms with one inlet and with an adjoining distribution line system to the forms which are characterized in that at least one static one in the distribution line system Mixer arranged according to at least one of claims 1 to 7 is. A hot runner block similar construction belongs to unpublished prior art (EP-OS 02 93 756). Among other things, it is used for simultaneous production of several preforms made of one PET material. In the Manufacture of preforms from PET material (poly ethylene terephthalate) the temperature of the pre - and material injected into the preform forms within a relatively narrow temperature range (see A Layman's Guide to PET Chemistry and Processing; Edward E. Denison, Eastman Chemical Products, Inc., Presented at  the 4th Annual International Conference on Oriented Plastic Containers, March 25, 1981). If the temperature is too low, the crystalline content in the material is too high. To strive for is, the entire material is possible in amorphous Transfer state.

On the other hand, the temperature is too high, causing the remaining would reduce the crystalline fraction, so too much Acetaldehyde is formed in the material, which is known to be a Is flavor. This can be very small Proportions affect the taste of a product that in bottles stretched from the preforms or containers is filled.

Therefore, on the one hand, complete avoidance is more crystalline Proportions in the PET molded into the preforms Material not possible. But are material shares with locally increased crystalline content in the preform forms sprayed, this can cause the preforms to become opaque Lead streaks, because PET material with increased crystalline Share whose crystal structures only under the influence of heat emerged, is opaque. The streaking is at known manufacturing process a problem which how mentioned, not simply solved by increasing the temperature can be.

In the known hot runner block (EP-OS 02 93 756) one as such to the pre-published prior art belonging flow divider in the form of two intersecting and helically wound transverse walls in the direction of flow provided, which, however, only serves to uniform within the viscous plastic melt To ensure temperature distribution while at the hot runner block designed according to the invention at the same time  a constant heat transfer from the block into the plastic melt he follows.

The invention is based on in the drawing reproduced embodiments of the invention. In the drawing shows

Fig. 1 (a) - (d) several embodiments of a static mixer in a schematic representation, each in a perspective view and in section;

Fig. 2 is a perspective view of another embodiment of the static mixer;

Fig. 3 is a plan view and a section IV-IV of a preferred embodiment of a hot runner block and

Fig. 4 is a schematic representation of the arrangement of a static mixer on the line system outside a hot runner block.

In Fig. 1 (a) to (d) four examples of line sections are shown having a substantially circular internal cross-section 11 of.

According to Fig. 1 (a), a limited in its extent in the direction of the conduit axis A pin-shaped flow divider 13 is a transversely extending and preferably centrally over the cross section 11 is provided. Referring to FIG. (B) the internal cross-section 11 is divided by a flow divider 13 b 1 which does not extends through the entire cross-section 11. A preferred embodiment of a flow divider 13 c is shown in FIG. 1 (c). It divides the circular cross-sectional area 11 into circular sectors 17 , preferably into three of the same size. In this embodiment variant, the spoke-shaped pins 14 protrude into the center, where they are connected to one another in an area 16 . Several identical or different flow dividers according to FIGS. 1 (a) to (d) can be connected in series. In a preferred embodiment according to FIG. 2, two or more sets of three spokes, offset azimuthally, are provided one behind the other and at a predetermined distance on the line section, wherein they can be connected to one another to facilitate installation. This embodiment has an axially extending support 19, followed, axially spaced, spokes 14 a, which preferably three, and preferably evenly distributed in a plane projecting. Two or more such sets of spokes are connected in series so that their spokes are offset by an angle α in the azimuthal direction.

The line sections constructed according to FIGS. 1 and 2 can easily be classified within the framework of existing line systems. The flow dividers 13 a to 13 c can be formed in one piece with the line 12 , or an insert part for forming the flow divider can be produced and subsequently installed.

In all of the aforementioned embodiments, the flow dividers consist of a good heat-conducting material, e.g. B. steel, and are thermally connected to the line 12 .

in Fig. 3 is a plan view and a cross section through a hot runner plate 20 of a hot runner block. Such hot runner plates, which are completed by applying cover plates to the hot runner block on both sides, are used for the simultaneous production of preforms from a PET material. The hot runner plate 20 has a number of outlet openings 22 , for the production of, for example, 32 preforms, at the same time 32 such outlet openings 22 . The mold for the preforms is mounted on these outlet openings 22 .

The hot runner plate 20 has a central inlet 24 , from which, with equal lengths, distribution channels 26 extend to the outlet openings 22 or lead to their immediate area when the union of these distribution channels 26 with the preform form only at the hot runner plate 20 in the flow direction F downstream organs. Molten PET material is driven through the inlet 24 and the distribution channels 26 against the outlet openings 22 and finally to the preform molds (not shown). The PET material is then pre promotes hot and cool down when passing through the hot runner plate 20 prevented by providing a meander shape distributed heating coils 28 on both surfaces of the hot runner plate twentieth The heating spirals 28 are operated electrically.

As mentioned in the introduction, the way in which PET material is processed must be given particular attention to its heating. If the pre-conveyed PET material is too warm only locally, districts with an increased crystalline content form there. On the other hand, if the material is heated too much, even locally, there is an increased formation of acetaldehyde. However, if areas with an increased crystalline fraction form due to insufficient local heating and if such areas emerge from a preform shape, coaxial with the outlet openings 22 , opaque streaks may result on the preform produced there.

In order to ensure that areas with an increased crystalline content in the otherwise amorphous PET mass, which can hardly be avoided, emerge locally concentrated at one or the other outlet opening 22 , according to FIG. 4, in which, based on the representation of FIG. 3, schematically the channels 26 to the openings 22 on the hot runner plate 20 are shown, basically a static mixer may be provided. As indicated at 30 in FIG. 4, this can already happen in front of the hot runner plate 20 and / or, as shown schematically in dashed lines at 32 and otherwise preferred, along the distribution channels 26 in the hot runner plate 20 .

In the preferred embodiment according to FIG. 4, flow dividers 32 , as shown in FIG. 2, are inserted into the channels 26 . In this case, these flow dividers are shown in FIG. 2, having azimuthally staggered spokes 14 a, for example, axially supported in the channels 26 by means of plug 34.

The temperature distribution in the hot runner plate 20 is stationary with a correspondingly long operating time, but is in no way homogeneous in that the temperature distribution from the heating meanders 28 as heat sources decreases towards the edges of the hot runner plate 20 , where it must satisfy the boundary conditions there. Thus, in certain areas, the channel walls of the channels 26 are warmer than at locations with greater distances from the heating meanders 28 . It follows from this that the material flow lying there in the region of the channel walls even in the case of turbulence and excited axial equalizing flow is exposed to higher temperatures and an increased amount of acetaldehyde can form there locally. In order to achieve a homogenization of the material stream, as far as the supplied heat quantity or the temperature in particular in those regions, the heat from the channel walls to the inside conductive in the flow of material flow divider 32 are inserted in particular those, the Heizmäandern nearest channel regions. By the end faces 36 shown in FIG. 2 are adapted exactly to the inner walls of the channels and thus a good heat transfer, z. B. is generated by a press fit, such a heat flow is achieved from the channel wall into the interior of the flowing PET material.

The table below shows the acetaldehyde values on 32 preforms produced simultaneously without the installation of flow dividers in the line system of the hot runner block and below that the measured values on a 32 preform set produced simultaneously, but under the same conditions, but with a flow divider according to FIG. 2 in section 38 ( Fig. 3 ) was generated.

It can be seen from this that a single flow divider in the channel section 38 already results in a 30% reduction in the acetaldehyde content of the preforms.

Table 1

Table 2

In absolute terms, modern extruder screw systems, upstream of the hot runner, can be used to obtain lower acetaldehyde levels. The relative relation, values with flow divider according to FIG. 2 / values without this flow divider, remains valid.

Claims (8)

1. Static mixer for mixing a flowing liquid with a tubular line through which the liquid flows and which preferably has a circular inner cross section and with at least one substantially radially oriented pin-like flow divider in the line, characterized in that the flow divider is made of a good heat-conductive material exists and is thermally connected to the wall of the line to reduce temperature differences between the wall and the liquid. 2. Mixer according to claim 1, characterized in that the pin-like flow dividers arranged like spokes are. 3. Mixer according to claim 1 or 2, characterized in that the pin-like flow dividers have three spokes exhibit.   4. Mixer according to one of claims 1 to 3, characterized characterized in that the flow dividers made of metal, preferably steel. 5. Mixer according to one of claims 1 to 4, characterized characterized that the flow divider on the wall the line are attached. 6. Mixer according to one of claims 1 to 5, characterized characterized that two sets of spokes offset azimuthally are provided that are centrally spaced apart are connected. 7. Mixer according to one of claims 1 to 6, characterized characterized in that the liquid is a viscous Plastic melt is high temperature. 8. Hot runner block for tempering and distributing one Plastic melt higher temperature to a variety of forms with an inlet and with one itself subsequent distribution line system to the Forms, characterized in that in the distribution line system at least one static mixer at least one of claims 1 to 7 is arranged.